Machine learning approaches are widely used in geosciences. However, one widely available dataset in reservoir geology remains underrepresented in published works: petrographic data from classical point-counting analyses. Such data are widely available for reservoir lithology characterization, often in combination with routine core analysis data (porosity and permeability). Since porosity and permeability in siliciclastic rocks are controlled by the detrital and authigenic composition and samples record effects of compaction during diagenesis, these datasets are often linked to assess reservoir quality controls.Datasets from six wells, covering four regions and two large reservoir lithologies in central Europe, the Permian Rotliegendes and Triassic Buntsandstein, were used to apply machine learning to the petrographic and reservoir quality data to predict porosity and permeability. Predictions are based on point-counting data including detrital and authigenic phases, optical porosity, grain-to-IGV (GTI) and grain-to-grain (GTG) coating coverages, and granulometry. For both regression tasks, a Random Forest and a Support Vector Regression machine learning model were implemented, with performance compared and the best model selected based on coefficient of determination (R2) and error metrics. Porosity predictions using a Random Forest algorithm yielded an R2 of 0.92, a mean average error (MAE) of 1.25%, and a root mean square error (RMSE) of 1.56%. Permeability predictions of real-scale permeability using Support Vector Regression gave an R2 of 0.85, MAE of 29.4 mD, RMSE of 68.3 mD, and a range-based normalized RMSE of 8.76% (real-scale). Log-transformation of measured and predicted permeability resulted in a more representative R2 of 0.83, MAE of 0.21, and RMSE of 0.24, reflecting its log-normal distribution. Predictions are acceptable despite the limited dataset, which reduces operator bias by using curated data. This machine learning approach may simultaneously unlock another understanding of reservoir quality controls based on SHapley Additive exPlanations (SHAP) value plots.Further training of such models on cored reservoir sections can improve understanding of which detrital and authigenic mineral phases influence reservoir properties. Trained models could also potentially evaluate reservoir properties from cuttings, which, like well logs, are more continuous than cores while allowing diagenetic interpretation based on petrographic analysis.

Carbon Capture and Storage (CCS) is a promising approach that has been increasingly utilized for reducing atmospheric carbon dioxide (CO2) levels. The long-term security of CO2 sequestration in saline aquifers depends on the integrity of overlying sealing units, while the transmissive properties of multi-barrier sealing systems remain to be further elaborated. This study investigates the permeability of claystones and compares them with other low-permeability lithologies (evaporites, tight sandstones, and tight limestones, 1 × 10− 17 − 1 × 10− 21 m² at 30 MPa) as well as casing cements (1 × 10− 16 − 1 × 10− 22 m² at 30 MPa) under CO2 storage conditions. Permeability was measured using helium gas and, for selected samples, with CO2. The stress-sensitivity of Klinkenberg-corrected permeability was assessed under confining pressures of 10–30 MPa, and pressure sensitivity coefficients (γ-values) were determined. Time-dependent effects were evaluated by maintaining samples at 30 MPa (approximately 2 km depth of effective pressure) confining pressure for 5–8 days. Results indicate that claystones reduce permeability by approximately one order of magnitude after 3–5 days, while casing cements decrease permeability by two orders of magnitude in 3 days. These findings highlight the necessity of allowing caprock samples to equilibrate under targeted effective stresses for at least three days to obtain reliable permeability measurements. Furthermore, our measured permeability varies distinctly across lithotypes (shales, evaporites, tight sandstones, and tight limestones), spanning 2 to 6 orders of magnitude at low confining stresses, necessitating the site- and lithotype-specific assessment of sealing unit permeability.

Machine learning techniques are increasingly applied in geological research and widely adopted in industry. However, one commonly available dataset remains underutilized: petrographic data from classical point-counting analyses. These data, routinely collected for reservoir lithologies worldwide, are often paired with core measurements such as porosity and permeability and capture detrital and authigenic components, textural properties, and diagenetic effects that largely govern reservoir quality. Building on an initial proof of concept, we expand the scope to a legacy dataset comprising 875 samples from 51 wells, compiled over 25 years by at least 21 petrographers. This dataset demonstrates the feasibility of predicting porosity and permeability from point-counting data across diverse lithologies and sources. Despite potential operator bias and classification inconsistencies, predictive performance remains robust. We present the outcome from two Histogram-based Gradient Boosting Regression Tree models trained on four major reservoir lithologies in Germany and the Netherlands: Upper Carboniferous, Permian Rotliegendes, Triassic Buntsandstein, and Jurassic sandstones. The porosity model achieves R2 = 0.87, MAE = 1.77%, and RMSE = 2.23%. The permeability model (log-transformed) yields R2 = 0.82, MAE = 0.47, and RMSE = 0.64, consistent with the log-normal distribution of permeability. SHAP analyses highlight key petrographic features influencing predictions, offering insights into detrital and diagenetic reservoir quality controls. Model performance remains robust under well-wise splits, confirming applicability to unseen wells. Training on cored intervals may enable extension to cuttings, which are more continuously available along well sections. Leveraging such legacy datasets can enhance reservoir quality assessment in sample-limited projects and improve the understanding of global reservoir systems.

Underground Hydrogen Storage (UHS) is a promising approach to store energy in large quantities to balance the fluctuation between renewable energy supply and overall energy demand. Compared to established underground natural gas storage, UHS imposes stricter requirements on gas tightness and long-term well integrity, which must be thoroughly investigated before implementation. While thermodynamic modelling suggests potential chemical reactions relevant to long-term stability, experimental data remain limited. Additionally, no studies have investigated polymer-modified cements, which exhibit promising material properties to meet the increased requirements. This study investigates the reactivity of hardened wellbore cement paste and polymer-modified cement paste with hydrogen under conditions of 50 degrees C and 100 bar over 4 weeks. To examine the individual effects of gas, temperature, and pressure on the reactivity, a combination of microstructural and mineralogical analyses, along with mechanical and physical properties evaluations, was conducted under various exposure conditions. The results indicate no detectable reactivity between hydrogen and the tested cement materials, supporting the feasibility of long-term and safe UHS using wellbore cements.

Understanding structural and diagenetic interactions is key in analyzing flow pathways in tight lithologies in the focus of geo-energy production as e.g., geothermal energy. Fracture spacing and clustering is crucial for reservoir production success and reduces uncertainty in reservoir exploration and utilization. This outcrop analog study evaluates diagenesis, fractures, vein cement generations, kink bands, and faults in tight fractured limestones of the Upper Muschelkalk-Lower Keuper transition on the eastern Upper Rhine Graben shoulder in SW Germany. Early and burial diagenesis led to cementation of former pore spaces and therefore drastically reduced the matrix porosity and permeability (porosity: 0.13-10.87 %, permeability: <0.0001 mD to 9.7 mD). Highest permeabilities are recorded in samples containing partially sealed veins and stylolites (up to 9.7 mD) at 1.2 MPa confining stress. The impact of increasing confining stress on the permeability of undisturbed limestones, as well as limestones containing stylolites and partially sealed veins, indicate that besides an undisturbed host rock sample and a sample containing stylolites, partially sealed veins preserve higher permeability at 30 MPa confining stress (41 % of initial value, compared to 16 and 11 % of the initial value). Fracture cluster analyses using the normalized correlation count method indicates that clustering around a breached kink band and associated fault is not symmetrically arranged and contains fracture sets of different strike. Fracture clusters also exist away from the fault at the breached kink band. Slip and dilation tendencies indicate that clustered fracture sets striking NNE-SSW parallel to the Rhine graben rift, WNW-ESE, and NW-SE parallel to the in-situ maximum principal horizontal stress are more likely to contribute to fluid flow as they are suitably oriented in the present-day stress field. Breached, decameter-scale reverse kink bands are the first reported in the region, c. 180-200 km N-NNE of the Alpine deformation front. Kink bands are most likely related to compression by far field stresses induced by the Alpine orogeny during the Eocene, and show partially cemented fault planes indicating locally persevered pore space. The applied methods of structural and diagenetic reservoir quality assessment and obtained outcomes aid in the understanding of fluid migration pathways for geoenergy applications in the Upper Rhine Graben area. Further, the results are also transferrable to other fractured tight reservoirs worldwide, which can help to solve problems for energy or heat supply that are of societal importance.

Reservoir quality of the Lower Triassic Buntsandstein in the Upper Rhine Graben (URG) is currently being explored to better assess the possibility of geothermal production and associated lithium brine production. Previous studies highlight a generally positive effect of increased detrital grain size and blocky cement content on elevated porosity and permeability, while outlining a generally negative influence of compactional processes, which are enhanced by tangential illite grain coatings and to a lesser extent by ductile rock fragments. This overall assessment can be supported by the studied samples from a research well (Kraichgau-1002) on the western graben shoulder, penetrating the coarser-grained Lower Buntsandstein at present-day depths between 500 and 710 m. The overall porosity and permeability is low (2.9 to 16.0 %, <0.0001 to 7.8 mD) in the studied core section. Tangential illite grain coatings enhance the effect of chemical compaction, whereas blocky cements (quartz, K-feldspar, carbonates, anhydrite) stabilize the grain framework against mechanical compaction. In samples with the same porosity in this study, higher permeability is found in samples with a coarser grain size. However, when also including other available Buntsandstein core samples, the overall correlation between permeability and grain size is poor (R2=0.05). Furthermore, a reported correlation between clay mineral grain coating characteristics (grain-to-IGV and grain-to-grain coating coverage) and detrital grain size could not be established (R2<0.01 and R2=0.19, respectively) for neither the studied samples, nor the combined Buntsandstein sample series from core material. Therefore, the detrital grain size by itself is an unsuitable criterion to assess Buntsandstein reservoir quality in the marginal basin facies. However, the extension of available datasets, which assess reservoir quality controls in the Buntsandstein in the area surrounding the URG highlights, that the previously defined reservoir quality controls are affecting larger areas of the marginal facies of the Buntsandstein present in NE France and SW Germany.

The petrophysical properties (porosity and permeability) of rocks are significantly influenced by their microstructure and fabric anisotropy, which can be evaluated using X-ray micro-computed tomography (mu CT). Through study of mica schists from the Singhbhum Shear Zone (Eastern India), we demonstrate the potential to perform X-ray micro-CT studies in metamorphic rocks and discuss the associated challenges in data processing. We show that automated thresholding of greyscale values from mu CT data acquisition yields abnormally high porosity/permeability values in schists when compared to the values obtained from laboratory measurements of the same samples. We develop a modus operandi where the laboratory measured porosity value from an individual schist sample is used to calibrate the greyscale threshold range (designating void space) of mu CT data from the same sample. This calibration is done using PACE3D numerical simulation framework that allows a multiphase-field approach, and it is shown that (a) porosities of schist derived from analysis of mu CT data (post rethresholding) fit well with laboratory measured values of respective samples and (b) anisotropy of permeability can be computed from mu CT data. Permeability computed using the mu CT data (post re-thresholding) vis-a`-vis laboratory measurements are comparable in 4 out of 5 schist samples analysed here, when the samples are treated as a two phase system (void spaces as one phase and solid rock mass as the second phase) in PACE3D. The aberration in one schist sample is attributed to its heterogeneous layering and microstructure that comprises alternate layers of coarse and fine grain size aggregates of phyllosilicate + quartz. Re-computation of permeability by performing three-phase simulations for the above layered sample in PACE3D framework by introducing phyllosilicate as a third phase (in addition to void spaces and other mineral phases) yields results similar to the laboratory measurements. We conclude that our approach of integrating mu CT data, laboratory measurement of petrophysical properties and microstructure modelling/simulation in PACE3D multiphase-field framework helps evaluate the role of rheological variations in controlling porosity/permeability. Thus this study has a bearing on enhancing knowledge about fluid flow in metamorphic rocks with possible implications for mineralization.

Joint petrophysical, geochemical, and petrographic studies on Barremian carbonate rocks from the Gagra-Java Zone at the transition of the Central Greater Caucasus to the Rioni Basin in Georgia are rare and almost absent in literature. In a joint research project between Georgian and German research institutes, this work showcases the first consistent dataset on rock properties, which are the foundation of the Enguri High Arch Dam, a large and regionally important energy infrastructure. The studied carbonate rocks from a 307 m deep research well are subdivided into two carbonaceous and two dolomitic sections, exhibit intense brecciation, represented by multiple rubble zones. Petrographic analyses in conjunction with petrophysical sample analyses do not show a clear correlation of rock properties to the rock's microfabric. Generally, matrix permeabilities are low to very low (< 10 mD) and do not correlate with matrix porosities. Elevated matrix permeability is only found in samples from the topmost 120 m of the studied well, but is not restricted to samples exhibiting carbonate mineral dissolution (mostly dolomite dissolution), which can be observed along the whole studied well. Based on cathodoluminescence analyses eight distinctly different phases of carbonate cement precipitation in brecciated sections and fractures can be distinguished and related to faulting and compressive tectonic phases resulting in the formation of stylolites. The analyses are supplemented by first UCS measurements (ranging from 19.9-175.0 MPa) to gain a better understanding on the mechanical properties of the studied samples.

Drill cuttings, though rarely used, are crucial subsurface samples to understand petrographic properties affecting reservoir quality. Unlike core material, cuttings are continuously available along the wellbore and can be used during drilling to monitor progress. Therefore, cuttings may allow a semi-quantitative, statistical calibration of rock properties from the subsurface, but they are often underutilized. Although fracture and vein orientations cannot be reconstructed from drill cuttings, the presence of veins and their internal textures (open, partially sealed or sealed) in specific formation sections and depths can be identified and analyzed using e.g., transmitted light microscopy and cathodoluminescence to supplement characterization at the well site and subsequently assess production behavior. Borehole gamma ray logs in combination with handheld portable X-ray fluorescence (pXRF) analyses on cleaned and dried drill cuttings can be used to further improve the depth accuracy of the cutting samples and to geochemically fingerprint the samples, based on the Si/Al ratio, as a proxy for sandstone-rich and mudrock-rich sections of the well. In this study, eighty-three sandstone cutting samples from two wells, covering ∼400 m of stratigraphy targeting the Paleocene-Eocene Greifenstein Fm. equivalent (Glauconite Sandstone, GLS) in the Vienna Basin (Austria), were studied. They also cover parts of three different reservoir sections (1. to 3. GLS). The Flysch play in the Vienna Basin hosts several sandstone-mudrock interbeds and is composed of several nappes, forming complex reservoir compartments. The glauconite contents vary between different sections of the GLS, where the highest is observed in the 3. GLS. The sandstones are predominantly cemented by ferroan calcite, resulting in low optical porosity (<5 %) in both wells, with only individually elevated porosity, related to partially dissolved K-feldspar grains. A paragenetic sequence solely based on cuttings further highlights that reservoir quality in the studied section is independent of sandstone compaction, but is related to lower optical porosity in finer-grained sandstones and higher carbonate vein cement contents. Furthermore, productive intervals are related to lower Fe + Mg contents. The understanding of reservoir properties, diagenesis, and their influence on fluid flow is crucial for successful exploration and reduction of uncertainty in reservoir production and development. The diagenetic variations from cuttings and the geochemical fingerprint by pXRF are linked to reservoir quality and production performance of individual well perforations. This approach can provide additional information on reservoir quality where core material is unavailable.

The presence of clay coatings on the surfaces of quartz grains can play a pivotal role in determining the porosity and permeability of sandstone reservoirs, thus directly impacting their reservoir quality. This study employs a multiphase-field model of syntaxial quartz cementation to explore the effects of clay coatings on quartz cement volumes, porosity, permeability, and their interrelations in sandstone formations. To generate various patterns of clay coatings on quartz grains within three-dimensional (3D) digital sandstone grain packs, a pre-processing toolchain is developed. Through numerical simulation experiments involving syntaxial overgrowth cementation on both single crystals and multigrain packs, the main coating parameters controlling quartz cement volume are elucidated. Such parameters include the growth of exposed pyramidal faces, lateral encasement, coating coverage, and coating pattern, etc. The coating pattern has a remarkable impact on cementation, with the layered coatings corresponding to fast cement growth rates. The coating coverage is positively correlated with the porosity and permeability of sandstone. The cement growth rate of quartz crystals is the lowest in the vertical orientation, and in the middle to late stages of evolution, it is faster in the diagonal orientation than in the horizontal orientation. Through comparing the simulated results of dynamic evolution process with the actual features, it is found that the simulated coating patterns after 20 d and 40 d show clear similarities with natural samples, proving the validity of the proposed three-dimensional numerical modeling of coatings. The methodology and findings presented contribute to improved reservoir characterization and predictive modeling of sandstone formations.

Underground hydrogen storage in porous rocks is a promising method to stabilize renewable energy fluctuations. However, data on the geochemical reactivity of hydrogen with reservoir rocks and its potential effects on reservoir performance are limited. This study investigates the geochemical reactivity of hydrogen with Buntsandstein reservoir sandstones from northern Germany, collected at a depth of about 2.5 km. Experiments were performed at 100 degrees C and 150 bar hydrogen partial pressure for four weeks, examining scenarios with dry hydrogen, synthetic saline fluid with hydrogen, synthetic saline fluid with helium (as a control), and an oxidation environment (air). We measured permeability, porosity, magnetic susceptibility, and fluid element concentration before and after the experiments. Results showed no significant mineral changes attributed to hydrogen. Magnetic susceptibility indicated no formation of magnetic minerals, such as magnetite and pyrrhotite. Minor variations in permeability and porosity were attributed to anhydrite dissolution from fluid chemistry nonequilibrium. Overall, our findings suggest hydrogen interactions with Buntsandstein sandstone (no pyrite content) at temperatures up to 100 degrees C do not risk hydrogen loss or reservoir performance degradation.

The cessation of hard coal mining in the Ruhr Basin in 2018 marked the region's transition to the post-mining phase. Controlled mine water rebound induces changes in the subsurface stress conditions, as pore pressure increases locally. Presently, mine water rebound is observed in the eastern Ruhr Basin (water province “Haus Aden”) along with associated microseismicity. Furthermore, post-mining challenges might comprise the potential risk of fault reactivation, which is addressed in this study by conducting a fault slip assessment.Based on subsurface coal seam mapping data, a 3D structural model for the NE part of the “Haus Aden” water province has been constructed to serve as the basis for identifying the most vulnerable fault trends and types of the structural inventory. Slip tendency analysis, considering normal faulting conditions, revealed NW-SE to NNW-SSE trending normal faults to be most susceptible to reactivation. Probabilistic fault slip assessment, focused on NW-SE to NNW-SSE trending normal faults mapped within the “Heinrich-Robert” colliery, show no fault reactivation potential for a mine water rebound up to a level of 640m below ground. Assuming hydrostatic conditions in the vicinity of the faults, friction coefficients are only partially exceeded for high differential stresses.In addition, a novel workflow is used to model the spatial variability of the frictional fault strength as input for a fault stability analysis, exemplified for a selected NNW-SSE trending normal fault. For considering hydrostatic pore pressure, results show that the fault consists mainly of stable, but also unstable, horizontally elongated patches. These findings question the conventional simplified approach of using a single constant friction coefficient for fault stability analysis.

Reservoir quality (RQ) in the Buntsandstein of the Upper Rhine Graben is in the center of attention as it is a possible target formation for geothermal energy production and hydrocarbon exploration surrounding existing fields. An understanding of properties affecting reservoir quality of the target lithology is still fairly poor and the success of accurately targeting high RQ intervals is limited. This is due to the fact, that the effect of compaction and the interplay between enhanced chemical compaction (i.e. pressure dissolution of quartz grains), illitic contact coatings and quartz cementation in the lithology has been underestimated. The understanding and quantification of controlling factors on reservoir qualities in fluvio-eolian sedimentary rocks, deposited in a (semi-)arid climate has been improved in recent years and this case study highlights the benefits of detailed petrographic analyses in understanding diagenetic and compactional processes in this lithology. This is especially relevant in the observation of grain coating clay minerals, whose effect depends on their specific location, i.e. either at grain contacts between quartz grains (GTG coating) or at the interface between detrital quartz grains and the intergranular volume (GTI coating).Illitic GTI coatings affect syntaxial quartz overgrowth precipitation, as precipitation sites are locally blocked. The negative correlation between the grain coating coverage and quartz cement volumes support these findings across multiple sample sets, and they may locally preserve intergranular porosity. Illitic GTG coatings on the other hand enhance chemical compaction (i.e. pressure dissolution) and will reduce the IGV. The negative correlation between these two properties again underlines the negative effect of this process on reservoir properties. Studied samples from a deep Buntsandstein well in the central URG show low reservoir quality due to either intense quartz cementation (0.7–31.7%) or a high degree of mechanical and chemical compaction (IGV: 2.3–38.0%). Higher illitic GTG and GTI coating coverages play a substantial role in controlling reservoir quality development, as demonstrated by comparing data from other fluvio-eolian lithologies (Triassic Buntsandstein and Permian Rotliegendes) to results of this study. In relation to their respective burial histories, higher illitic GTI coating coverages always correlate with smaller syntaxial quartz cement volumes. Similarly, higher illitic GTG coating coverages always correlate with lower IGV values in the three compared sample series.As both, the precipitation of quartz cements and compaction, are a function of the burial history, i.e. effective stresses and experienced temperatures, understanding the interaction of both these processes may enable the prediction of reservoir properties in undrilled areas.

Foreland basins have been extensively explored for hydrocarbon resources, geothermal, and gas storage applications, and multidisciplinarily studied to reconstruct orogenic processes. In fact, in the case of the Late Carboniferous Central European Variscan foreland basin, numerous drill cores have been recovered by the hard coal industry and comprehensive geological data sets have been made available by research. These cores and data sets provide a continuous record of the tectono-sedimentary and reservoir evolution over time. For this purpose, we have compiled petrophysical and petrographic literature data of >450 mostly tight (porosity <10%, permeability <1 mD) siliciclastic rock samples from Variscan foreland (Ruhr and Lower Saxony basin, NW Germany) and intramontane basins (Saar-Nahe basin, SW Germany) covering the complete Late Carboniferous stratigraphy. As a result, increases in mean grain size, sorting, detrital quartz, and proportion of metamorphic to sedimentary rock fragment contents, and decreases in detrital feldspar contents along the Westphalian B–C boundary are interpreted as a response to sedimentary recycling and unroofing of the Variscan hinterland due to tectonic uplift in the course of the northwestward propagating Variscan front. Thus, tectonics have a greater influence on sandstone petrography and porosity than the depositional environment. Porosity is mainly controlled by grain size and dissolution porosity. Basin-specific paleogeothermal gradients affect authigenic quartz (up to 25.0%) and chlorite (up to 7.7%) formation. Moreover, quartz cement contents <5% may stabilize the granular framework against mechanical compaction preserving porosity. With increasing quartz cement contents >5%, however, pore space is progressively clogged and the porosity preserving effect diminishes.

Former coal mines hosted in Upper Carboniferous silt- and sandstones in the Ruhr Basin, NW Germany, are currently examined for post-mining applications (e.g. geothermal energy) and are also important tight-gas reservoir analogs. Core material from well Pelkum-1, comprising Westphalian A (Bashkirian) delta deposits, was studied. The sandstones and siltstones are generally tight (mean porosity 5.5%; mean permeability 0.26 mD). Poor reservoir properties primarily result from pronounced mechanical compaction (mean COPL 38.8%) due to deep burial and high contents of ductile rock fragments. Better reservoir properties in sandstones (>8%; >0.01 mD) are due to (1) lower volumes of ductile grains (<38%) that deform during mechanical compaction and (2) higher volumes in feldspar and unstable rock fragments. During burial these form secondary porosity (>1.5%) resulting from acidic pore water from organic matter maturation. Still, sandstones with enhanced porosities only show a small increase in permeability since authigenic clays (i.e. kaolinite and illite) or late diagenetic carbonates (i.e. siderite and ferroan dolomite/ankerite) clog secondary porosity. Quartz cementation has a minor impact on reservoir properties. Evaluating the Si/Al ratio can be a suitable proxy to assess grain sizes and may be a convenient tool for further exploration. Supplementary material: Lithologs and petrophysical data of well Pelkum-1 are available at https://doi.org/10.6084/m9.figshare.c.7003156

Summary Micromodels play a significant role in investigating flow, transport, interaction of various substances within synthetically fabricated micro-spaces, typically constructed from artificial materials such as PDMS, glass and silicon. Micromodels also show substantial potential for addressing critical challenges of the energy transition, e.g., CO2 or hydrogen flow properties, mineral dissolution/precipitation, and microbial activities in porous rocks. However, the rarity of real-rock micromodels that incorporate both the original pore geometry and minerals poses a limitation. In this study, we present a novel approach to natural-rock micromodels that combine microfluidic fabrication and thin section techniques, using natural sandstones. The key strength of the method lies in its ability to include rock chemistry and grain surface morphology (e.g., roughness and clay/hematite coating), features often absent in conventional micromodels. Additionally, it enables real-time pore-scale resolution under a microscope, a capability not achievable with core samples. Our micromodel currently focuses on sandstones (silicates), providing a new opportunity to investigate the interaction between gas, fluids, and particularly microbes. We are currently shedding light on the potential interaction between methanogenic archaea and minerals as well as pore geometry under hydrogen storage conditions. This method may provide new possibilities for studying reactions correlated to rock minerals and microstructures.

The Upper Cretaceous limestones unconformably overlie Upper Carboniferous coal-bearing lithologies and are studied to assess their effect on rising mine-water levels in the Ruhr mining district. Upper Cretaceous sedimentary rocks from the Münsterland Cretaceous Basin have previously been studied regarding their sedimentary structures and fossil content. However, understanding the petrophysical and petrographic heterogeneity in regard to sedimentary properties and their effect on fluid migration pathways is yet missing. Utilizing He-pycnometry, Klinkenberg-corrected air permeabilities, p-wave velocities, transmitted and reflected light analyses, point-counting and cathodoluminescence, we assess the petrophysical, geomechanical and mineralogical properties. Porosity ranges from 1.0 to 18.7

Deltaic siltstones and sandstones from the Pennsylvanian (Upper Carboniferous) in the Ruhr Basin are currently being examined for post-mining applications (e.g., geothermal) but are also an important tight-gas reservoir analog in NW Germany. Core material from two wells in the eastern Ruhr Basin, comprising Bashkirian delta deposits of the Langsettian and Duckmantian substages (Westphalian A and B), were studied using petrographic and petrophysical data to assess their reservoir properties and factors controlling these. The samples have low porosities and permeabilities (mean porosity 5.5